Technical Field
The present invention relates to a light emitting device applicable to purposes such as an indicator, a lighting apparatus, a display, a backlight light source for liquid crystal display, and a method of manufacturing the light emitting device.
Description of Background Art
In recent years, various semiconductor devices have been proposed and are put in practical use, and the demand for high performance is ever increasing. In particular, electric components are required to have high reliability to maintain performance for a long period of time, in other words, to maintain stable operation for a long period of time, even under severe environment. The same applies for light emitting diodes (LEDs) and other light emitting devices. Requirements for higher performance in the area of general lighting, in-vehicle lighting, and the like, is growing daily, and further higher output (higher luminance) and higher reliability are demanded. A further demand is to supply such a device at a low price while fulfilling those requirements.
In order to achieve higher output power, it is efficient to improve optical output efficiency of the light emitting element (semiconductor light emitting element) which is to be used. For a method to improve the optical output power of the light emitting element, for example, a method using a small chip dice (light emitting element) is employed (for example, see Patent Literature 1). Particularly, in the case where a gallium nitride-based LED is used for the light emitting element, the light emitted from the light emitting element propagates in the semiconductor layer, and therefore is absorbed when it is reflected at the electrode or the like (for example, see Patent Literature 2). For this reason, the dice is made as a small chip to allow the emitted light to be extracted to outside. Thus, absorbtion loss of light can be decreased. In the case where a small chip dice is used, a limited amount of electric current is allowed to flow through. Thus, employing a multi-dice structure having a plurality of small chips enables to obtain a desired optical output efficiently.
In addition, for a structure of the light emitting element, a flip-chip type may be employed in which the electrode surface to be electrically connected to an external electrode is arranged downward (hereinafter may be referred to as a face-down element or FD element). (see Patent Literatures 1, 3). This structure does not have an electrode and/or wire etc., on the principal light extracting surface of the light emitted from the light emitting element. Thus, it is said to be able to further improve the optical output efficiency (extracting efficiency).
Also, in order to improve the optical output power of the light emitting element, a silver plating which has a high reflectance is typically applied on the electrically conductive member used for the base member. On the other hand, as the material of the base member, in the fields of general lighting, in-vehicle lighting, and back light light source, ceramic materials which are resistant to deterioration under high temperature and high optical density are typically used (for example, see Patent Literature 1).
Futher, gold (Au) is typically used as a conductive wire used for a protective element or a face-up element (hereinafter may be referred to as a FU element) having an electrode surface arranged over the light emitting element. Au wire is very soft and a ball bonding technique can be used, so that very thin wires of ·100 μm or less, for example (diameter) of several tens of micrometers can be used. Thus, in the case where a plurality of light emitting elements are mounted, a plurality of conductive wires can be used.
Typically, a light emitting device has a base member (a package, i.e. a mount substrate having a wiring pattern etc.) on which electrical components such as a light emitting element and a protective element are mounted, and electrically conductive members which supply electric current (electric power) to such electric components. The light emitting device further has a sealing member protecting the electric components from external environment. However, loss due to absorption of light (absorption loss of light) occurs depending on the materials of the base member, electrically conductive members, sealing member, etc. Particularly, the surface area of the electrically conductive members is relatively large, absorption loss of light by the electrically conductive members may reduce light extraction efficiency. For higher output power, the light extraction efficiency to be improved, and for this, improvement in the optical output efficiency of the light emitting element (semiconductor light emitting element), and also reduction of absorption loss of light by the materials of base member (including the package), electrically conductive members, and sealing member are effective.
In order to improve the light extracting efficiency, for example, application of a plating of a metal member having high reflectance on the inner surface of the package to suppress absorption of light by the base member and to efficiently extract light to the outside is proposed (see Patent Literature 4).
Disposing of a member having high reflectance in the light emitting device is viewed as a way to suppress optical absorption by the members used in the light emitting device, but among those materials having high reflectance (such as silver), some materials are subjected to sulfuration or halogenation and may cause longterm reliability concerns. That is, there has been a problem in which, discoloration due to sulfuration or halogenation of the material by the sulfur component etc., contained in the atmosphere causes reduction of the reflectance of the material, which results in reduction of the light extracting efficiency.
In order to solve such a problem, Patent Literature 5 discloses forming of titanium dioxide (TiO2) etc., as a protective film on the surface of a metal reflection film by using sputtering or vapor deposition to improve its gass barrier property. Also, Patent Literature 6 discloses a reduction of the problems caused by heat, by a reflector free of discoloration formed by covering the reflecting surface of the reflector with a high-reflecting resin layer made of a powder material having high reflectance mixed with a resin, and by employing a material having excellent heat dissipating property for the reflector.
Also, known is a light emitting device in which, other than that as described above, a light emitting element is mounted in a flip-chip mounting manner as described in Patent Literature 7. The light emitting element includes a transparent substrate such as sapphire, and a semiconductor layer stacked thereon. In the light emitting element, each electrode is bonded on the lead pattern through a respective conductive bump. With this arrangement, the transparent substrate side of the light emitting element can be utilized as the light extracting side. Further, a light emitting device in which a lower portion and a side surface are covered with a resin containing a filler (Patent Literatures 8 to 11), and a technique of electrodeposition of titanium dioxide on the light emitting element (Patent Literature 12).
Patent Literature 1: JP 2009-135485A
Patent Literature 2: JP 2008-112959A
Patent Literature 3: JP 2005-150484A
Patent Literature 4: JP 2006-156603A
Patent Literature 5: JP 2006-351964A
Patent Literature 6: JP 2007-281260A
Patent Literature 7: JP 2005-210051A
Patent Literature 8: JP 2004-172160A
Patent Literature 9: JP 2007-109948A
Patent Literature 10: JP 2007-19096A
Patent Literature 11: JP 2009-130237A
Patent Literature 12: JP 2004-158843A